1. Field of the Invention
The present invention relates to a technique for controlling a display time point of an MPEG bit stream, and more particularly to an apparatus and method for controlling a display time point of an MPEG bit stream of a recording medium which is capable of controlling accurately a display time point of an MPEG bit stream in case that an MPEG bit stream of a recording medium is processed to be displayed according to a special decoding command and returns to a normal decoding command.
2. Description of the Background Art
FIG. 1 is a schematic block diagram of an apparatus for controlling a display time point of an MPEG bit stream of a broadcasting medium in accordance with a conventional art.
As shown in the drawing, the apparatus for controlling a display time point of an MPEG bit stream of a broadcasting medium of the conventional art includes a counter 40 being initialized according to a system time reference (SCR), for receiving a system clock frequency (27 MHz), counting it and outputting a system time clock (STC); a subtractor 10 for receiving the system time clock (STC) and the system time reference (SCR) from the counter 40, subtracting them and outputting a difference signal (e); a low-pass filter and amplifier 20 for receiving the difference signal (e) from the subtractor 10, low-pass filtering it and amplifying it; a voltage control oscillator 30 for receiving the output signal from the low-pass filter and amplifier 20, controlling a voltage of the output signal, and outputting a system clock frequency (27 MHz); and a comparator 50 for receiving presentation time stamps (PTS) of a predetermined broadcasting picture and the system time clock (STC) outputted from the counter 40, comparing them, and outputting a display command signal in case that a time point of the system time clock (STC) and a time point of the presentation time stamps (PTS) are identical to each other upon comparison.
The operation of the apparatus for controlling display time pint of an MPEG bit stream of a broadcasting medium of the conventional art constructed as described above will now be explained with reference to the accompanying drawings.
To begin with, after the counter 40 is initialized according to a system clock reference (SCR), it receives a system clock frequency (27 MHz) from the voltage control oscillator 30, counts it and outputs a system time clock (STC).
That is, when bit streams are inputted at a constant rate, the counter 40 initializes the decoder clock on the basis of the system clock reference (SCR) inputted from an encoder (not shown).
At this time, the subtractor 10 receives the system time clock (STC) and the system clock reference (SCR) from the counter 40, obtains a difference signal (e) between the two signals, and transmits it to the low-pass filter and amplifier 20.
Upon receipt of the difference signal (e) from the subtractor 10, the low-pass filter and amplifier 20 low-pass filters it, amplifies it to a predetermined level and then outputs it.
Thereafter, the voltage control generator 30 receives the output signal (f) of the low-pass filter and amplifier 20, controls its voltage and outputs a system clock frequency (27 MHz).
Meanwhile, the comparator 50 receives the system time clock (STC) from the counter 40 and the presentation time stamps of a broadcast picture and compares them. Upon comparison, if the time point of the system time clock (STC) and the time point of presentation time stamps (PTS) are identical to each other, the comparator 50 outputs a display command signal, according to which a predetermined broadcast picture is displayed on a screen.
Namely, the apparatus for controlling a display time point of an MPDG bit stream of a broadcasting medium in accordance with the conventional art is based on the assumption that the MPEG bit streams are inputted at a constant rate, so that the system time clock can be computed on the basis of the system clock reference that is periodically inputted to the apparatus.
However, as for the apparatus for controlling a display time point of MPEG bit streams of a recording medium, the MPEG bit streams are not inputted at a constant rate. Thus, the system clock reference can not be used to obtain a system time clock of the decoder.
In addition, in case of decoding the recorded MPEG bit streams, it is not necessary to recover a decoder clock in synchronization with a clock of the encoder as shown in FIG. 2. However, the MPEG bit streams should be displayed by using the system clock reference and the presentation time stamps at a predetermined time. In this case, there is a problem that, in the course of performing special decoding, such as a pause, a fast winding or slow motion, if it returns to perform a normal decoding, the system time clock and the presentation time stamps are asynchronized.